The effect of LiCl on the stereochemistry of the anionic polymerization of methyl methacrylate in the presence of a lithium counterion at -78-degrees-C in THF and in a 9/1 toluene/THF mixture, has been ... [more ▼]

The effect of LiCl on the stereochemistry of the anionic polymerization of methyl methacrylate in the presence of a lithium counterion at -78-degrees-C in THF and in a 9/1 toluene/THF mixture, has been studied by C-13 and H-1 NMR spectroscopy. It has been found that LiCl has an important effect on the E/Z ratio of the living chain ends in THF, in contrast to the main-chain tacticity. In contrast, LiCl has a significant influence on the stereoregularity of the anionic polymerization of MMA in a 9/1 toluene/THF mixture. When the LiCl/initiator molar ratio is increased, the syndiotactic placements (rr) are favored at the expense of the isotactic ones (mm). This effect can be accounted for by a multistate propagating mechanism controlled by association/complexation equilibria among associated, nonassociated, and LiCl-complexed species. These association/complexation equilbria, which were originally proposed by Muller, appear to have a decisive effect not only on the livingness (molecular weight distribution (MWD)) but also on the stereochemistry of the anionic polymerization of MMA in a 9/1 toluene/THF mixture at -78-degrees-C, confirming the involvement of the ligand in propagation control. [less ▲]

In this study we report on the adsorption behavior of end-attaching triblock and diblock copolymers. The triblock copolymers consist of relatively short PEO end blocks and a much longer PS middle block or ... [more ▼]

In this study we report on the adsorption behavior of end-attaching triblock and diblock copolymers. The triblock copolymers consist of relatively short PEO end blocks and a much longer PS middle block or, in one case, of zwitterionic end groups with a PS middle block. The diblock materials consist of a PEO block of fixed length and varying polystyrene lengths. A silicon wafer with an oxide layer is used as the surface, and adsorption takes place from toluene; only the end groups adsorb, leaving the polystyrene block dangling in solution. We find that diblock copolymers which range from symmetric to moderately symmetric obey the scaling law for the surface coverage which was proposed in the theory of Marques and Joanny (i.e., sigma is-proportional-to 1/N(A), where N(A) is the number of segments in the adsorbing block). However, we find that for triblock copolymers of comparable asymmetry the surface coverage scales according to the relationship sigma is-proportional-to 1/beta2, where beta is the ratio of the size of the nonadsorbing block to that of the adsorbing block. This implies that moderately symmetric triblock copolymers behave like highly asymmetric diblock copolymers; the adsorption behavior is dominated by the nonadsorbing block size. Our experiments demonstrate the observation of extremely long-lived nonequilibrium states. On the basis of these observations we propose a simplified version of a recent theory of adsorption which is capable of qualitatively describing much of the experimentally observed phenomena. [less ▲]

The surface compositions of a series of random copolymers of styrene and deuteriostyrene have been determined by SIMS. The mechanism of fragmentation during the SIMS process is examined, and the ... [more ▼]

The surface compositions of a series of random copolymers of styrene and deuteriostyrene have been determined by SIMS. The mechanism of fragmentation during the SIMS process is examined, and the contribution from naturally occurring isotopes is evaluated. The data are shown to fit a model of a completely random copolymer system if they are corrected for the effects of impurities introduced in the syntheses and for the contribution from C-13. [less ▲]

Ring-opening polymerization of epsilon-caprolactone has been initiated with functional aluminum alkoxides in toluene at 25-degrees-C. Degradation of ''living'' polyester chains by transesterification ... [more ▼]

Ring-opening polymerization of epsilon-caprolactone has been initiated with functional aluminum alkoxides in toluene at 25-degrees-C. Degradation of ''living'' polyester chains by transesterification reactions has been studied in relation to temperature, solvent polarity, average molecular weight, and structure of the initiator, i.e., the nature of the functional group and the number of alkoxides per Al atom. Degradation products, particularly cyclic oligomers, have been isolated by supercritical fluid extraction (SFE) and characterized by GC-MS. The effect of Lewis bases on transesterification reactions has been investigated and discussed in reference to the ''coordination-insertion'' mechanism. [less ▲]

Diethylaluminum dialkoxides (Et2AlOXOAlEt2, where X is either an aliphatic radical or a polymer chain) and related dithiolates have been investigated as potential initiators for the ring-opening ... [more ▼]

Diethylaluminum dialkoxides (Et2AlOXOAlEt2, where X is either an aliphatic radical or a polymer chain) and related dithiolates have been investigated as potential initiators for the ring-opening polymerization of ε-caprolactone. Hydroxy telechelic poly(ε-caprolactone) and alpha,omega-dihydroxypoly(ε-caprolactone-b-X-b-ε-caprolactone) have been accordingly prepared and characterized. A hydroxy group is selectively attached at both ends as demonstrated by IR and H-1 NMR spectroscopies and confirmed by titration of the derivatized alpha,omega-dicarboxylic acid (co)polymers. Polymerization of epsilon-caprolactone is living and yields linear polyester chains of a predictable molecular weight and of quite a narrow molecular weight distribution. [less ▲]

A monomeric model structurally comparable to the active end of a living poly(methyl methacrylate) (PMMA), i.e., a methyl α-lithioisobutyrate (MIBLi)/THF solution, was studied by both Li-7 and C-13 NMR. It ... [more ▼]

A monomeric model structurally comparable to the active end of a living poly(methyl methacrylate) (PMMA), i.e., a methyl α-lithioisobutyrate (MIBLi)/THF solution, was studied by both Li-7 and C-13 NMR. It has been shown that two different coordination aggregates exist in a MEBLi/THF solution. That situation corresponds to a slow exchange process between tetramer and dimer, i.e. the later being favored by a decrease in temperature or concentration. Such an equilibrium is characterized with ΔH = -2.1 kcal/mol and ΔS = -13.7 cal/mol.K by means of Li-7 NMR. Again, C-13 NMR results obtained suggest that a MIBLi/THF solution displays a charge-delocalized character, which is different from a O-Li-bonded bonded structure in solid state established by crystal analyses of several similar ester enolates. [less ▲]

Whenever added with triethylaluminum, primary amines have been found to be effective initiators for epsilon-caprolactone polymerization in both toluene and THF at 40°C. The IR and NMR analysis of the ... [more ▼]

Whenever added with triethylaluminum, primary amines have been found to be effective initiators for epsilon-caprolactone polymerization in both toluene and THF at 40°C. The IR and NMR analysis of the polyester has supported a polymerization mechanism proceeding through a "coordination-insertion" pathway and the selective rupture of the acyl-oxygen bond of the monomer. The alkylaluminum activates the carbonyl group of the monomer and accordingly favors the nucleophilic addition of the amine, which is the actual initiation step. Propagation is typically a living process, and the molecular weight distribution is controlled by both AlEt3/amine molar ratio and solvent As an extension of that mechanism, diethylaluminum omega-amino alkoxide has been prepared on purpose and successfully used as an initiator for the synthesis of α,ω-dihydroxypoly(ε-caprolactone). [less ▲]

Tailor-made poly(tert-butyl acrylate) macromonomers, which are useful building blocks for the synthesis of comb or multiblock copolymers, have been successfully synthesized by deactivation of living PtBA ... [more ▼]

Tailor-made poly(tert-butyl acrylate) macromonomers, which are useful building blocks for the synthesis of comb or multiblock copolymers, have been successfully synthesized by deactivation of living PtBA macroanions with suitable electrophiles bearing either styrene or methacrylic unsaturation. Mono- or bifunctional initiators such as sec-butyllithium or naphthalenyllithium can be used. When these initiators were modified by LiCl as a mu-type ligand, the macroanions which are known to be living were functionalized with high efficiency. Characterization of the various macromonomers was performed using H-1 NMR spectroscopy and SEC. [less ▲]

The synthesis and use of functional diethylaluminum alkoxides (Et2AlORX) as initiators for the ring-opening polymerization of ε-caprolactone have been investigated as a possible strategy to prepare omega ... [more ▼]

The synthesis and use of functional diethylaluminum alkoxides (Et2AlORX) as initiators for the ring-opening polymerization of ε-caprolactone have been investigated as a possible strategy to prepare omega-primary amino poly(ε-caprolactone) (PCL). In a first approach, an initiator containing ethylphthalimide as the functional-RX group has been synthesized. That protected amino group has been successfully attached at the end of PCL chains. The conversion of the phthalimide end group into a primary amine has however failed, whatever the experimental conditions. In an alternative approach, an alkyl bromide has been considered as the functional group to be attached to PCL chains, via the appropriate initiator. The end functionalization was quantitative and the bromo end group converted into an azido group which was subsequently reduced to the expected primary amine. Omega-primary amino PCL has proved to be an effective macroinitiator for the ring-opening polymerization of γ-benzylglutamate N-carboxy anhydride with the formation of a poly(caprolactone-b-peptide) diblock copolymer. [less ▲]

Copolymerization of mixtures of epsilon-caprolactone (epsilon-CL) and D,L- or L-lactide has been initiated by aluminum isopropoxide. On the basis of the monomer reactivity ratios and assuming that no side ... [more ▼]

Copolymerization of mixtures of epsilon-caprolactone (epsilon-CL) and D,L- or L-lactide has been initiated by aluminum isopropoxide. On the basis of the monomer reactivity ratios and assuming that no side reaction occurs, tapered copolymers should be formed particularly when D,L-lactide is the comonomer rather than L-lactide. The sequential distribution of the comonomers has been analyzed by an original method based on the quantitative measurement of the carbonyl C-13 signals. Comparison of the experimental and the theoretical distributions clearly shows that transesterification reactions occur during copolymerization through both intra- and intermolecular mechanisms. Gel permeation chromatography and differential scanning calorimetry data support that conclusion. The D,L-lactide-based copolymers have however a more blocky structure than those containing L-lactide, and they are also less sensitive to transesterification reactions. [less ▲]

An alpha,omega-bis(dimethylamino)polyisoprene (M(n)BAR = 5000) has been quaternized by alpha,alpha'-dibromo-p-xylene with formation of a model cationic ionomer. Dielectric spectroscopy has shown an alpha ... [more ▼]

An alpha,omega-bis(dimethylamino)polyisoprene (M(n)BAR = 5000) has been quaternized by alpha,alpha'-dibromo-p-xylene with formation of a model cationic ionomer. Dielectric spectroscopy has shown an alpha-relaxation which corresponds to the micro-Brownian motion of chain segments directly attached to the quaternary ammonium dipoles. A shoulder on the low-temperature side of the alpha-relaxation has been assigned to the contribution of the polymer segments, the mobility of which is not restricted by the multiplets. That situation is in full agreement with the multiplet-cluster concept recently revisited by Eisenberg et al. [less ▲]

Poly(methyl methacrylate)-b-poly(tert-butyl acrylate) (PMMA-b-PtBA) copolymers have been successfully synthesized by the sequential anionic polymerization of methyl methacrylate (MMA) and tert-butyl acrylate (tBA) in THF at -78-degrees-C. Although the order of monomer addition makes a difference on the initiation efficiency, it has no significant effect on the final achievement. Mono- and bifunctional initiators based on alkali metals can be used. When the initiators are modified by LiCl as a ligand, the polymerization of each block appears to be living, the molecular weight and composition can be predicted, and the molecular weight distribution is narrow. Size exclusion chromatography supports the absence of homo-PtBA in the PtBA-b-PMMA samples. The PtBA blocks can be quantitatively hydrolyzed into polyacid ones as supported by titration and H-1 NMR analysis. [less ▲]

Aluminum isopropoxide is known to be an effective initiator for the ring-opening polymerization of lactides (D,L and L,L) and epsilon-caprolactone (epsilon-CL). For a question of mutual reactivity, the ... [more ▼]

Aluminum isopropoxide is known to be an effective initiator for the ring-opening polymerization of lactides (D,L and L,L) and epsilon-caprolactone (epsilon-CL). For a question of mutual reactivity, the sequential polymerization of these two comonomers can only be achieved when epsilon-CL is first polymerized followed by the lactide. Formation of a large amount of homo PLA is however observed and has been attributed to a great difference in the mean degree of association of aluminum alkoxides in toluene in the presence of epsilon-CL and LA. In toluene, the mean number (n) of active sites per Al(O(i)Pr)3 molecule jumps from 1 to 3 when epsilon-CL is substituted by LA. The addition of a small amount of an alcohol, like 2-propanol, is effective in preventing the aluminum alkoxide from associating, and n is then 3 whatever the monomer used. 2-Propanol participates in the polymerization of epsilon-CL and LA as supported by a decrease in the molecular weight of the final polymer in relation to the molar amount of added alcohol. This means that the alcohol molecules are rapidly exchanged with the alkoxide groups on Al. Under such conditions, the formation of pure diblock copolymers P(epsilon-CL-b-LA) is reported. Another successful strategy for the synthesis of the diblock copolymer consists in using an Al derivative that bears only one alkoxide group to initiate the polymerization of the two cyclic esters. In this regard, a diethylaluminum alkoxide is of great value, particularly the ethoxide one. A third, conceptually similar, approach to the synthesis of P(epsilon-CL-b-LA) copolymers relates to the reaction of a performed hydroxy-terminated PCL with triethylaluminum and the use of that macroinitiator in the ring-opening polymerization of the lactide. [less ▲]

Aluminum isopropoxide is an effective initiator for the polymerization of lactides in toluene at 70-degrees-C. The ring-opening polymerization proceeds through a "coordination-insertion" mechanism and ... [more ▼]

Aluminum isopropoxide is an effective initiator for the polymerization of lactides in toluene at 70-degrees-C. The ring-opening polymerization proceeds through a "coordination-insertion" mechanism and selective rupture of the acyl oxygen bond of the monomer. A kinetic study has shown that the polymerization is first order in both the monomer and initiator. The polymerization is typically "living" until a molecular weight of ca. 90 000 is reached. At higher temperatures (100-degrees-C), inter- and intramolecular transesterification reactions are responsible for a limitation in the molecular weight of the growing chains. In toluene, at 70-degrees-C, each alkoxide of the aluminum isopropoxide participates in the polymerization. Any molecular weight below 90 000 can accordingly be predicted on the basis of the monomer/Al molar ratio, and the polydispersity is rather narrow (1.1-1.4). [less ▲]

Well-defined omega-methacryloylpoly(epsilon-caprolactone) (or PCL) macromonomers have been prepared by using an aluminum alkoxide that bears a methacrylic double bond. The initiator results from the ... [more ▼]

Well-defined omega-methacryloylpoly(epsilon-caprolactone) (or PCL) macromonomers have been prepared by using an aluminum alkoxide that bears a methacrylic double bond. The initiator results from the equimolar reaction of triethylaluminum with 2-hydroxyethyl methacrylate (HEMA). The coordination-insertion type of polymerization is living and yields exclusively linear macromonomers of a predictable molecular weight with a narrow molecular weight distribution. IR and H-1 NMR studies show that the methacryloyl group associated with the active alkoxy group of the initiator is selectively attached to one chain end. Should the living polymerization be terminated by methacryloyl chloride, and a alpha,omega-dimethacryloyl-PCL, i.e. a alpha,omega-macromonomer is obtained. PCL macromonomers and HEMA are easily copolymerized in the presence of AIBN, resulting in a hydrophilic poly(HEMA) backbone grafted with hydrophobic PCL subchains. [less ▲]